The last generation of storage rings is designed to operate with low emittances (longitudinal and transverse) and rather high currents. Performances of these machines are limited by the collective effects. The interaction of a beam with the surrounding walls produces parasitic fields which, acting back on the beam, can lead to instability phenomena. Other instability effects are known, where the beam interacts with foreign (ions, electrons, dust,etc.). Recently, new types of instabilities have drown the attention of accelerator physicists.